1. Technical Field
The present invention relates to a shock-absorbing fence that absorbs impacts of falling objects such as falling rocks and soil, and in particular relates to a shock-absorbing fence that has the improved capability to capture falling rocks.
2. Description of the Related Art
In Japanese Patent Application Laid-Open Publication No. H7-197423, there is disclosed a shock-absorbing fence that includes: tubular main posts built in vertical holes bored at intervals on a hillside slope to be provided in a standing state; and a net formed of metallic mesh provided in a stretched state on each of the tubular main posts along with cables at a plurality of stages.
There is also proposed a shock-absorbing fence for absorbing impacts of falling rocks and the like by converting the impact energy into the friction energy, which includes as the basic configuration: main posts provided to stand at predetermined intervals; horizontal rope members anchored between the main posts in a horizontally slidable manner and secured at both ends thereof; and a wire net retained on the horizontal rope members to cover a space between the main posts. In addition to the basic configuration, the proposed shock-absorbing fence further includes as an absorbing section: an extra length section formed by overlapping rope members in the middle of the horizontal rope members; and a clump that clumps the extra length section with a certain strength. In case a tensile force beyond the predetermined level is applied to the horizontal rope members, the extra length section can extend to absorb the tensile force while a certain friction force is maintained on the horizontal rope members (see for example, Japanese Patent Application Laid-Open Publication No. H6-173221, and Japanese Patent Application Laid-Open Publication No. H6-336709).
(1) This is a type of shock-absorbing fence configured to stop falling rocks and the like in a band-shaped net stretched across main posts. When such a shock-absorbing fence catches falling rocks, the midsection of the net deflects and deforms by a large amount toward the foot of a slope, and at the same time, the height (effective height) of the fence changes by a large amount.
In other words, the net deflects and deforms by a large amount in and around the region hit by falling rocks, and due to such deformation, the length between the upper and lower rims of the net (net width in its height direction) becomes shorter.
As a result, in case that rockfalls occur in a repetitive manner, rocks from subsequent rockfalls can slip through a gap wide open between the bottom end of the net and the ground, and the rocks may jump over the net, posing problems in its capability to trap falling rocks.
(2) If cost is not taken into account, it is possible to enhance capability to trap falling rocks by reducing the amount of deformation by utilizing a large quantity of absorbers, increasing the number of ropes, and installing a multiple layer of shock-absorbing fences.
However, in light of the current severe economic environment, it is difficult to implement costly countermeasures as mentioned above, and for this reason, it is much needed to propose a shock-absorbing fence whose installation costs can be lowered without sacrificing the protection performance.